Store ForeignItem in a side table.

compiler/rustc_plugin_impl/src/build.rs

@@ -25,6 +25,8 @@ impl<'v, 'tcx> ItemLikeVisitor<'v> for RegistrarFinder<'tcx> {
     fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem<'_>) {}
 
     fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem<'_>) {}
+
+    fn visit_foreign_item(&mut self, _foreign_item: &hir::ForeignItem<'_>) {}
 }
 
 /// Finds the function marked with `#[plugin_registrar]`, if any.